Who is your friend – fat or carbohydrate?

Diet – a familiar word, and one which is dreaded by many. There are a large number of reasons for controlling food intake, and an equally large number of diets to choose from. For example, diets low in carbohydrates are thought to promote rapid weight loss. Insulin, which is secreted to mediate glucose export from the blood, promotes fat storage. It is believed that low insulin levels facilitate the release of fatty acids and fat oxidation and thereby lead to weight loss. These findings come from studies in which the subjects were prescribed particular diets. However, to what extent these diets were followed was assessed through a number of surveys, a typically unreliable method of collecting this data – after all, how many people would want to admit to breaking their diet?

A recent study by Hall et al investigating the effect of carbohydrate or fat restriction on body fat was the first of its kind to be conducted in a controlled-feeding manner on inpatients1. A group of clinically obese subjects were kept on a ‘baseline’ diet for five days, composed of 50% carbohydrate, 35% fat and 15% protein, before being divided into two groups. The first group was given a carbohydrate-restricted diet; the second one a fat-restricted diet. Both diets contained 30% less calories than the baseline diet. In the carbohydrate-restricted diet, the carbohydrate content was lowered to 29%. In the fat-restricted diet, the fat content was lowered to 8%. Crucially, all other macronutrients (such as protein) were kept constant, and calorie reduction was only achieved through reduced fat and carbohydrate intake in the respective diets.

The study found that the carbohydrate-restricted diet resulted in increased fat oxidation compared to the baseline diet, whereas the extent of fat oxidation did not change with the fat-restricted diet. However, the fat-restricted diet led to an overall greater fat loss than the carbohydrate-restricted diet at the end of a 6 day period. Those on the carbohydrate-restricted diet experienced a greater weight loss in comparison to the second group, although it was noted that this may be due to higher body water loss.

Hall’s close examination of the effects of a carbohydrate-restricted diet compared to a fat-restricted diet came to the conclusion that more body fat can be lost when eating foods with reduced fat content (with all other macronutrients kept the same), alongside an overall reduced calorie intake. However, one notable drawback of the study is that the relative hunger levels of participants on both diets were not monitored throughout. Typically, carbohydrate-restricted diets keep glucose and insulin levels relatively stable, which limits the feeling of hunger when dieting. On the other hand, a calorie-restricted diet that is high in carbohydrates might be difficult to follow in an uncontrolled setting. It is crucial for the success of a diet that it is easy to follow on a permanent basis. After all, a well-balanced diet in combination with regular exercise is probably the healthiest option. Keep this in mind when preparing your dinner tonight!